Process gases for use in semiconductor manufacturing facilities are generally supplied through cylinders. In order to meet the increasing demand for high flow rate product and ultra-high purity requirements for these gases, gas producers often use an ultra-high purity bulk vaporizer delivery system to treat polar liquefied compressed gases. In contrast, the proposed invention is an on-site system that uses a microwave source of power to increase and to control the evaporation rate of aqueous polar liquids (such as ammonia). The on-site system is able to provide purification, short response time, and accurately controllable vaporization at very high flow rates (exceeding 1000 liters/min in large volumes for the semiconductor industry). To sustain the vaporization of the polar liquid compound, power must be added to the system to replace the heat that is carried off with the gaseous flow to the customer use point. If this is not done, then, in the case of ammonia, the pressure and the temperature at the gas/liquid interface will drop and the polar liquid's vaporization rate will decrease until the gas-liquid pool eventually sub-cools and the vapor flow decreases to a negligible level.
The on-site vaporization system delivers power though a wave-guide and a quartz window. There is no solid source of contamination because there is no physical contact between the energy source and the polar liquid. As a result, the on-site vaporizer system reduces the amount of impurities that are introduced into the vapor phase, thus adding a purification step.
The impurities that will be excluded from the vapor phase and remain in the liquid pool as a result of liquid evaporation includes Group I, Group II and Group III metals, as well as oxides, carbonates, hydrides and halides of these elements. These impurities, for a conventional ammonia vaporizer involving vaporization using wall heating, come from a number of sources including friction from ammonia valves actuation, thermal expansion and contraction of liquid containers, pressure stretch or expansion on vessel container openings during filling, etc. Moisture is another impurity. Based on vapor-liquid equilibrium data, the concentration of moisture in the liquid phase is approximately 2 to 3 orders of magnitude greater than its concentration in the vapor phase. Careful handling in evaporating the liquid phase to the vapor phase can reduce moisture and other non-volatile residues (NVR) by orders of magnitude.
As illustrated in Table I, the semiconductor industry requires that critical impurities be removed from ammonia or other polar fluid. The critical impurities to be removed include moisture and NVRs such as oils and hydrocarbons. Generally, liquid ammonia feed contains 3,000 ppb moisture and 2,000 ppb NVR and oil. For semiconductor purposes, the invention's objective is to achieve less than 100 ppb moisture and 100 ppb NVR impurity level in ammonia or polar product fluid. Another objective is to achieve less than 5.0 ppb metals impurity level in ammonia or polar product fluid, although, we expect less than 1 ppt metal in the vapor product.
TABLE ICritical Impurity Levels Before and After PurificationImpurity Level in UHPImpurity LevelGaseous AmmoniaImpurityin FeedAcceptablePreferredMoisture3,000 ppb<100 ppb  10 ppbNVR2,000 ppb<100 ppb<10 ppbMetals   5 ppb <5 ppb <3 ppb
A common prior art practice for delivering purified gas is through the vaporizer approach, which uses electrical-resistance heating. The process withdraws polar liquid from a tank and heats it with a heat source from either an internal heater or external band heaters to provide heat in excess of available ambient heat. The vapor then passes through a heat exchanger located within the pool of polar liquid to promote further vaporization it the tank. Conduction and convective heat transfer to the bulk liquid from a line source increases the system response time. The use of immersion heaters or shell and tube heat exchangers promotes nucleus boiling, releasing vapor bubbles from the nucleation sites. Since the nucleation sites on the heating surface are substantially hotter than the bulk liquid vaporization of impurities, such as water, takes place. As temperature gradients and nucleation sites are generated, the agitation level in the bulk liquid is increased. Agitation also increases the mobility of NVR, increasing the chances of allowing passage into the vapor phase. As mentioned, the current invention avoids these problems because there is no physical contact between the power source and the liquid. With no immersion heater or heating through container walls, there are no surface hot spots, nor is there any appreciable attendant liquid agitation. Release and carry over of contaminating impurities is, thus, greatly reduced using the current invention.
U.S. Pat. No. 4,671,952 discloses a process and apparatus for generating sulfur dioxide vapor from contaminated liquid sulfur dioxide. The process uses contaminated liquid sulfur dioxide and subjects it to microwave energy at a frequency of 915, 2450, 5850 or 18000 Mhz for a sufficient period of time to produce sulfur dioxide vapor, collecting the vapor and removing the remaining contaminated liquid sulfur dioxide, The vapor pressure of the sulfur dioxide is 34.4 psig at 70° F. and purity of sulfur dioxide achieved is 98.99%. This patent does not teach or suggest the concept of vaporizing the liquid from the discrete penetration depth from the exposed liquid/vapor interface at the top of the liquid mass. Nor does it teach that this process will produce ultra-high purity vapor product. Certainly, there is no teaching or suggestion for segregating the heated liquid from the bulk liquid.
U.S. Pat. No. 4,285,774 discloses an apparatus that continuously produces concentrated alcohol from beer. A plurality of concentrator cells and a plurality of salvage cells are arranged in a line in side-by-side relation. Beer is supplied to the first upstream concentrator cell through a supply conduit. The beer then flows through passages between adjacent cells in response to the volume of beer reaching a predetermined level in the adjacent upstream cell. A microwave ignition bulb is positioned in each cell to heat the beer and boil or vaporize the alcohol content. The gaseous alcohol serially bubbles through a fluid passage from each concentrator cell to the next adjacent upstream cell until the gaseous alcohol reaches the first concentrator cell where the gaseous alcohol is concentrated and condensed in a column to a liquid solution containing approximately 95% alcohol and approximately 5% water. The alcohol obtained from the dilute, substantially spent beer in the salvage cells is collected and returned to the supply conduit for recycle. This invention purposely boils and vaporizes the alcohol in the liquid beer feed. To contrast, the instant invention avoids vaporizing the bulk of the liquid feed in order to increase the purity level of the gaseous product.
U.S. Pat. No. 5,882,416 discloses a liquid delivery system for delivering a liquid reagent in vaporized form to a chemical vapor deposition reactor. The reactor is arranged in a vapor-receiving relationship to the liquid delivery system. The liquid delivery system includes an elongated vaporization fluid flow passage defined by a longitudinal axis and bounded by an enclosing wall. Vaporization is achieved using a heating element contained within the fluid flow passage transverse to the longitudinal axis for heating the fluid to vaporization. The vaporized liquid is then carried to a chemical vapor deposition reactor.
U.S. Pat. No. 5,846,386 discloses an on-site vaporizer that draws ammonia vapor from a liquid ammonia reservoir. The ammonia vapor then passes through a microfiltration filter, and is then scrubbed using high-pH purified water. Commercial grade ammonia converts to sufficiently high-purity ammonia without the need for conventional column distillation. Liquid ammonia is stored in a reservoir. An external immersion heat source generates vapor from the liquid ammonia supply reservoir. Such vaporization constitutes a single stage distillation, leaving certain solid impurities and high-boiling impurities behind in the liquid phase. The ammonia vapor drawn from the vapor space in the reservoir passes through a microfilter. A pressure regulator controls the flow of the filtered vapor and directs it to a scrubbing column/circulation pump combination and then to either a distillation column, a deionized water dissolving unit for purified liquid product point of use, or to transfer lines for gaseous point of use. The vapor headspace of the reservoir controls the flow rate. A circulation pump is employed in the vaporizer system, which can be a source of metallic impurities.
U.S. Pat. No. 5,523,652 discloses using microwave energy in a dielectric plasma chamber, a pair of vaporizers, a microwave tuning and transmission assembly and a magnetic field generating assembly. The chamber defines an interior region in which a source gas is routed and ionized to form plasma. The microwave tuning and transmission assembly feeds microwave energy to the chamber in the TE10 (transverse electric) mode.
None of the prior art is believed to teach or suggest using microwave power to control vaporizing a polar liquid within the thermally segregated penetration depth of the liquid pool and produce ultra high purity gases.
As used herein, “penetration depth” (PD) means the depth of the liquid that is actually heated by the microwave power in this invention.
As used herein, “liquid depth” (LD) means the bulk liquid that is essentially unaffected by the microwave power.
As used herein, “flux” means the power (P) per unit of exposed fluid area (AS) exposed to the microwave energy (expressed in Watts/ft2).
As used herein, “freeboard” means the exposed fluid area exposed to the microwave energy.
As used herein, “ripple” or “disturbance” (RD) means the layer at the top of the penetration depth that has enough movement to start entering into the vapor phase, measured from the top of the crest to the bottom of the trough.
As used herein, “superheat” means the temperature above the boiling point of the liquid and reflects the thermal driving force (ΔT) required to achieve boiling condition for the liquid. As an illustration, if the boiling point is 25 degrees and the liquid is at 29 degrees, then the amount of superheat is 4 degrees.
As used herein, “agitation point” means the point where agitation of the liquid being vaporized begins. It is above the boiling point, the RD<0.5″ and preferably RD<0.1″.